Future infotainment systems in vehicles and the associated loudspeaker systems in vehicles have to fulfill challenging tasks in complex traffic scenarios. Therefore, absolutely reliable functioning is a prerequisite, wherein risks for the driver, for example by erroneous function, have to be avoided in any driving situation. Here, communication requirements and fast provision of information as well as undisturbed audio reproduction play an important role. Here, not only vehicle sounds are considered as spurious signals, but also parallel consumption of different audio content, such as when talking on the phone and consuming media content at the same time from the perspective of several passengers. Such challenges necessitate system characteristics allowing individual sound exposure of limited audio regions, so-called sound or listening zones.
Typically, apart from the electroacoustic components, efficient algorithms for noise suppression and effective data communication for regulating the adapted system are necessitated for realizing these systems.
Starting from this problem, several concepts exist that are used in the market and are at least partly proven, respectively. One example is the personalized sound exposure (by means of sound zones) by using loudspeakers in direct proximity to the ears of the listener in the respective sound zone, e.g. by loudspeaker integration in the respective headrests of the respective car seat per listening zone. Such a system with loudspeakers divided into groups is disclosed in the U.S. Pat. No. 8,126,159. One advantage of this approach is the high acoustic separation with respect to the adjacent sound zones due to the great difference in the listening distance. This is based on the theoretical model of level decrease of approximately 6 dB per duplication of the distance (with ideal spherical wave propagation). A disadvantage of this approach is the high sensitivity to disturbances, e.g. due to head movements. This results, on the one hand, in high level fluctuations and significant impediments of spatial perception, e.g. loss of the stereo images.
A second conventional approach concerns personalized sound zones that can be generated by using ultrasound technology. Listening sound is modulated to ultrasound carriers and radiated to the listening zone in a highly focused manner. A prerequisite of this modulation principle is the radiation of very high ultrasound levels, e.g. higher than 130 dB. The advantage of this approach is that the ultrasound, due to the favorable ratios of wavelength to size of the active “radiation area” defined by the size of the loudspeaker and the loudspeaker array, respectively, is radiated in a more focused manner than frequencies of the audio frequency range. Thus, increased acoustical separation of the sound zones is possible, while maintaining the size of the used loudspeaker technology. The disadvantage of this approach is not only that ultrasound can be unhealthy from certain power levels (see in this regard usage of ultrasound in the medical field for destroying kidney stones), but also that, when using ultrasound, strong reflections in the vehicle interior result, which have a disadvantageous effect on the acoustic channel separation. Further, ultrasound usage causes high power consumption, which is equivalent to low energy efficiency. Additionally, highly non-linear transmission behavior occurs due to the demodulation principle, resulting in low sound quality which is normally only sufficient for speech reproduction.
A further conventional approach is based on so-called beamforming. For this, several loudspeakers are used, which are, for example, distributed within the vehicle and/or are grouped into a loudspeaker array. By the specific control of each loudspeaker, directed sound radiation, e.g. for individual sound zones, is obtained. In this context, reference is made to U.S. Pat. No. 8,073,156 disclosing the usage of linear loudspeaker arrays in a vehicle. Thereby it is possible to focus a sound pattern to one or several positions in the vehicle. Patent document US 2012/0121113 discloses the usage of a further loudspeaker array in a vehicle including the respective controller. The advantage with respect to the first approach is a more stable sound zone, even with head movement. Further, no direct proximity of the seating position to a loudspeaker installation position is necessitated. Compared to the second approach, there is no risk potential due to the high sound pressure. Additionally, better sound quality can be obtained compared to this ultrasound approach. A disadvantage, however, is the obtainable sound focusing, frequently resulting in insufficient channel separation, in particular caused by the realizable array dimensions, the realizable sound transducer distances (distance from adjacent electroacoustic sound transducers) and the number of sound transducers per array. Additionally, the channel separation of previous beamforming approaches is lowered by the spatial acoustic influences in the vehicle, reflections and room modes, respectively.
Further, U.S. Pat. No. 7,343,020 discloses an automobile audio system with directional planar sound transducers for generating stereo or surround sounds individually for each passenger. US Patent 2003/0021433 discloses a loudspeaker configuration together with a signal processor for stereo channel generation for each passenger individually by using a central loudspeaker. EP Patent 2 143 300 B1 discloses a vehicle loudspeaker system with directional sound transducers directed to the respective seating positions (=listening positions). All three latter approaches from the US/EP patents have in common that insufficient channel separation or crosstalk can result due to the loudspeaker technology to be derived. Thus, there is the need for an improved approach.